The vitamin K cycle supports blood coagulation and is a major target for anticoagulation drugs. Coagulation factors require Dcarboxylation of glutamic acids for activation at sites of injury. The vitamin K dependent [unreadable] carboxylase (VKGG) is driven by the oxidation of a vitamin K cofactor. The vitamin K epoxide reductase (VKOR) regenerates this cofactor;the reducing equivalent comes from a thioredoxin-like partner. VKOR is the target of warfarin, the most commonly used oral anticoagulant for treating and preventing thrombosis diseases including deep vein thrombosis, pulmonary embolism, stroke, and myocardial infarction. Warfarin has a narrow therapeutic window due to the high risk of hemorrhage. The design of safer VKOR inhibitors was impeded by the complete absence of structural knowledge of VKOR. During the K99 phase, we determined the structure of a bacterial homolog of VKOR in association with its reducing partner, a thioredoxin domain (Li etal.. Nature 2010). The structure reveals a binding pocket for warfarin and a pathway of electron transfer from the thioredoxin domain to VKOR. For the ROO phase, 1) we will determine the crystal structures of VKOR in complex with warfarin and other coumarin drugs. We will also use purified VKOR proteins to study the biochemistry of VKOR catalysis and warfarin inhibition. The studies will provide the basis for rational drug design. 2) We will determine structures of reaction intermediates to elucidate the pathway by which electrons flow from the thioredoxin domain to VKOR. We will also study the function of human VKOR with its reducing partners. 3) We will conduct structural studies ofthe gamma-glutamyl carboxylase (VKGC). I have obtained a full-time, tenure-track assistant professor position in the Washington University at St. Louis. The school has an excellent research environment with well-equipped laboratories and top researchers. Future research in my group will use a combination of X-ray crystallography, protein biochemistry, and cell biology. My long-term goal is to understand the entire vitamin K pathway that sustains blood coagulation.